Programming the AY-3-8910 sound chip

Programming the AY-3-8910 Programmable Sound Generator (PSG) is a relative simple task. But since the Aquarius BASIC doesn't support any command's for INPUT / OUTPUT the task has to be solved by machinecode. The info on this page is therefor for the advanced programmer with some assembler knowledge.

Before we start programming the PSG, we have to know something about it's internal structure. It has 16 registers, which are used as followed:

Register \ bit		B7	B6	B5	B4	B3	B2	B1	B0
R0	Channel A Tone Period	8-Bit Fine Tune A
R1						4-Bit Coarse Tune A
R2	Channel B Tone Period	8-Bit Fine Tune B
R3						4-Bit Coarse Tune B
R4	Channel B Tone Period	8-Bit Fine Tune C
R5						4-Bit Coarse Tune C
R6	Noise period				5-Bit Period control
R7	Enable ( bit 0 = on, 1 = off )	IN/OUT		Noise			Tone
		IOB	IOA	C	B	A	C	B	A
R8	Channel A Envelope on/off, Volume			Env	volume
R9	Channel B Envelope on/off, Volume			Env	volume
R10	Channel C Envelope on/off, Volume			Env	volume
R11	Envelope Period	8-Bit Fine Tune Envelope
R12						4-Bit Coarse Tune Envelope
R13	Envelope Shape/Cycle					CONT	ATT	ALT	HOLD
R14	I/O Port A Data Store	8-Bit Parallel I/O on Port A
R15	I/O Port B Data Store	8-Bit Parallel I/O on Port B

The Aquarius has two I/O ports in order to programm the PSG. Port 0xF7 is used to select the register and port 0xF6 is used to read or write the selected register. In order to generate a tone on Channel A we do the following:

data	label	opcode	operand	comment
62, 8	start:	LD	A,0x08	; Select register #8
211, 247		OUT	(0xF7),A
62, 15		LD	A,0x0F	; Volume channel A full
211, 246		OUT	(0xF6),A
62, 0		LD	A,0x00	; Select register #0
211, 247		OUT	(0xF7),A
62, 93		LD	A,0x5D	; Write #93 into register #0
211, 246		OUT	(0xF6),A
62, 1		LD	A,0x01	; Select register #1
211, 247		OUT	(0xF7),A
62, 13		LD	A,0x0D	; Write #13 into register #1
211, 246		OUT	(0xF6),A
62, 7		LD	A,0x07	; Select register #7
211, 247		OUT	(0xF7),A
62, 62		LD	A,0x3E	; Enable output Channel A (0011 1110)
211, 246		OUT	(0xF6),A
201		RET		; Return to BASIC

Translated to BASIC that would be:

10 DATA 62,8,211,247,62,15,211,246
20 DATA 62,0,211,247,62,93,211,246
30 DATA 62,1,211,247,62,13,211,246
40 DATA 62,7,211,247,62,62,211,246
50 DATA 201
60 FORX=32000TO32032:READA:POKEX,A:NEXT
70 POKE14340,0:POKE1341,125:X=USR(0)

Note	Period	Note	Period
C	3421	F#	2419
C#	3228	G	2283
D	3047	G#	2155
D#	2876	A	2034
E	2715	A#	1920
F	2562	B	1892

The lower the period, the higher the note. In order to get a higher octave, just divide the period by two.

The registers 0 & 1 are used for the period of Channel A. To calculate the correct values for these registers you can use the following formula:

Register 1 = INT( period / 256 )
Register 0 = period - ( Register_1 * 256 )

Frequency = 111861 / Period

Now let's take it a little step further and try working with an effect. The following will generate a short decaying burst of noise, just like a gunshot:

data	label	opcode	operand	comment
62,11	start:	LD	A,0x0B	; Register 11
211,247		OUT	(0xF7),A
62,160		LD	A,0xA0
211,246		OUT	(0xF6),A
62,12		LD	A,0x0C	; Register 12
211,247		OUT	(0xF7),A
62,15		LD	A,0x0F
211,246		OUT	(0xF6),A
62,6		LD	A,0x06	; Register 6
211,247		OUT	(0xF7),A
62,15		LD	A,0x0F
211,246		OUT	(0xF6),A
62,7		LD	A,0x07	; Register 7
211,247		OUT	(0xF7),A
62,55		LD	A,0x37	; 00110111
211,246		OUT	(0xF6),A
62,13		LD	A,0x0D	; Register 13
211,247		OUT	(0xF7),A
62,0		LD	A,0x00
211,246		OUT	(0xF6),A
62,8		LD	A,0x08	; Register 8
211,247		LD	(0xF7),A
62,16		LD	A,0x10	; Enable Enveloppe Channel A
211,246		OUT	(0x246),A
201		RET		; Return to BASIC

Again, translated to BASIC:

10 DATA 62,11,211,247,62,160,211,246
20 DATA 62,12,211,247,62,15,211,246
30 DATA 62,6,211,247,62,15,211,246
40 DATA 62,7,211,247,62,55,211,246
50 DATA 62,13,211,247,62,0,211,246
60 DATA 62,8,211,247,62,16,211,246
70 DATA 201
80 FORX=32000TO32048:READA:POKEX,A:NEXT
90 POKE14340,0:POKE1341,125:X=USR(0)

The effect is generated by the Enveloppe register #13. It can make several effects which are graphical representated in the next table:

R13 Bits					GRAPHICAL REPRESENTATION OF ENVELOPE GENERATOR OUTPUT (E3 E2 E1 E0)
B3	B2	B1	B0	Decimal representation
Continue	Attack	Alternate	Hold
0	0	x	x	0,1,2,3
0	1	x	x	4,5,6,7
1	0	0	0	8
1	0	0	1	9
1	0	1	0	10
1	0	1	1	11
1	1	0	0	12
1	1	0	1	13
1	1	1	0	14
1	1	1	1	15
					EP is the Envelope Period, or the duration of 1 cycle. EP = R12*256 + R11

After running the last BASIC programm one can experiment with the enveloppe effects by poking the wanted effect on address 32037 and calling the USR(0) function. For example:

POKE32037,4:X=USR(0)
POKE32037,8:X=USR(0)